A battery is known to generate Joule heat by its internal resistance and current passing therethrough when charged and discharged. In order to suppress excessive rise in the temperature of a battery, many methods are being used for cooling a battery with cooled air flowing from a cooling fan and the like. While with general methods for cooling a battery a fan is actuated to cool a battery when the temperature of the battery which is measured exceeds a predetermined temperature, methods have been also proposed for controlling an airflow of a cooling fan based not only on the temperature of the battery but also on the ambient temperature of the battery and the heat value of the battery, to thereby reduce a temperature rise caused by delayed cooling of the battery and an energy loss caused by excessive cooling (see JP 2008-27888 A, for example).